In automobile engines, the lubricating system normally includes a filter designed to remove particulate impurities from the lubricating oil. The filter is located in the oil flow path just downstream of the oil pump so that any particulate impurities in the oil are trapped in the oil filter before they reach vital engine areas such as the bearing surfaces.
One of the most critical periods when filtration of the oil is required is when breaking in a new or rebuilt engine. During the break-in period, the engine's moving parts are rubbing against each other and wearing down any surface irregularities until the parts mesh smoothly with each other. As this is occurring, minute particles of metal are often worn or broken off from the contacting surfaces and are picked up by the moving oil. In addition, if there are any gaskets and/or seals that are disintegrating due to improper installation or defective materials, particles of these materials will also become caught in the oil stream. As the oil is continually pumped, these particles become trapped in the filter.
After the initial break-in period is completed, the old oil and oil filter are normally removed from the engine and are replaced with new oil and a new oil filter. For most engines, this period occurs after approximately 500 miles of driving. For racing engines, the period is usually much less. At the end of this short period, an engine is considered to be broken in since the rate of engine wear will have greatly decreased.
If one were to cut apart the old oil filter to inspect it at the end of the break-in period, one would be able to view all of the collected particles. By inspecting the type and quantity of particles, one could ascertain how much wear had taken place and possibly the location(s) where the wear had taken place. If an unusually large amount of metal or gasket/seal particles are present, this indicates that an engine component either is misaligned, defective or was improperly installed. Even if the engine seems to be operating correctly, the presence of a great number of particles in the filter indicates that a premature engine failure might be imminent.
For the average vehicle, an inspection of the oil filter's interior is not performed after the break-in period. This is due to the large amount of time and effort required to cut open an oil filter. Since the rate of premature engine failures is fairly low, the added time and expense are not considered warranted. In addition, since the inspection results may be inconclusive or only signal that a part may fail under certain adverse conditions, most engine builders do not want to needlessly alarm their customers.
The above noted disinclination to inspect the oil filter is not shared by racing car mechanics or others who are concerned with the reliability and long-term durability of the vehicle's engine. For a mechanic in charge of a racing car, knowledge of how the engine's parts have broken in is critical. For this reason, the engine's oil filter is checked frequently during and after the racing engine's break-in period. However, for the average person, it is impractical and extremely difficult to check the interior of a typical oil filter.